My 1992 Buck Century sometimes makes a whirring sound when I reach a speed of 45MPH or more. Sometimes it sounds steady or sometimes rythmic like "whir, whir, whir or whoop, whoop, whoop. It dosen’t do it while standing still, so I know it’s not the engine. It’s strickly a motion noise. Before the car refrains from making that noise for my mechanic’s ears, can anyone provide a clue for the cause? Is it the brakes or wheel bearings? Transmission problem?